The invention pertains to use of sulfate-selective highly basic anion exchange resins in combination with TRSS resins for removing or reducing the concentration of dissolved salts, including sulfates, contained in an aqueous fluid.
Thermally regenerable salt sorbent (TRSS) resins are hybrid resins which contain both weak acid and weak base groups that can form internal zwitterions. Both anions and cations can be removed from feed water with TRSS resins, then the resin can be regenerated simply by thermal treatment which releases the captured ions, thereby re-forming the internal zwitterions.
However, the presence of sulfate ions in the feed water can be problematic to treatment with TRSS resins. The capacity of such resins may be reduced when sulfate ions occur with calcium cations in the feed. Presumably the reason for the reduced capacity is that both calcium and sulfate ions are returned together in the regenerant fluid and calcium sulfate precipitates with the resin bed at the temperatures used for regeneration. One proposed solution to this problem is to pre-treat the feed with the conventional type 1 strongly sulfate-selective trimethylamine anion exchange resin or a less sulfate-selective type 2 resin, such as dimethylethanolamine, to preferentially remove the sulfate ions and exchange them with chloride ions from the exchange resin. The feed could then be treated with the TRSS resin. The spent regenerant from the TRSS treatment would be enriched in chloride ion which in turn could be used to regenerate the anion exchange resin. See U.S. Pat. No. 4,184,948.
However, we have found that the regeneration of the conventional basic trimethylamine or dimethylethanolamine anion exchange resins using the regenerant from the TRSS treatment is not efficient. This is due to the lower concentration of chloride ion in the spent regenerant from TRSS treatment compared to the chloride ion concentration in brine typically used to regenerate anion exchange resins. While not intending to be bound by any theory, it seems that the sulfate ions are bound too strongly to the anion exchange resin to be effectively regenerated in low concentrations of brine.
We have found that when using a spent regenerant from TRSS treatment to regenerate a sulfate-selective highly basic anion exchange resin, an anion exchange resin with a lower selectivity for sulfate than trimethylamine- or dimethyethanolamine-containing resins is advantageous.
As used herein, the term “elution” refers to the removal of ions, cations and/or anions, which have been loaded on to the resin during the absorption process. The term “regeneration” refers to restoration of the functional groups to the resin to the zwitterion on a TRSS resin, or replacement of the captured anions with anions in the regeneration fluid in an anion exchange resin. These operations are each thermally activated and essentially simultaneously occur. Therefore, elution will necessarily also involve regeneration.